Sesha Hari Vemuri

Synthesis of layered–layered 0.5Li2MnO3·0.5LiCoO2 nanocomposite electrode materials by the mechanochemical process and first principles study

Li2MnO3-stabilized LiCoO2 electrode materials were synthesized using the method of mechanochemical process. Li2MnO3 was prepared and the mechanochemical process was carried out with LiCoO2, which yielded the layered–layered integrated structure nanocomposites. X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy studies confirmed the structural integration of 0.5Li2MnO3·0.5LiCoO2 electrode materials. We also performed the high temperature heat treatment, where our 0.5Li2MnO3·0.5LiCoO2 electrode materials showed improvement in the discharge capacity (∼180 mA h g−1) with good cycleability. To obtain a physical insight into the performance of the nanocomposite structure, we carried out first principles calculations to obtain activation energy barriers of Li+ de-/intercalation, which suggested that utilizing both Li2MnO3 and LiCoO2 components can enhance the Li+ diffusion for the layered–layered integrated structure.

New analysis of yield stress on giant electrorheological fluids

A new universal yield stress scaling equation is proposed to accurately model experimental data for giant electrorheological (GER) fluids. This new equation expressed in modified Bessel function predicts both regions of polarization effect predominant in the low electric field strength applied and polar molecule-dominating GER behavior, as well as collapses the experimental data of yield stress in a single line for a broad range of electric field strengths.

Head-disk interface design in magnetic data storage

Superior electrical conductivity and thermo-mechanical response of graphene can significantly improve areal density in magnetic data storage. The current head media spacing in hard disk drives (HDD) is 7.2 nm, and replacing conventional carbon overcoat (COC) with graphene will drastically reduce head media spacing, increasing areal density to eight times its present value. A paradigm shift in HDD systems can also be achieved via selection of a combination of new lubricants and unconventional architecture of COC systems. Here, we evaluate the feasibility of graphene overcoat (GOC), by understanding GOC-lubricant interactions. We further introduce new alternative head-disk interface (HDI) designs consisting of buffer/lubricant layers (i.e., graphene/carbon nanotube (CNT) or fullerene/perfluoropolyether (PFPE)). These hybrids could further enhance tribological performance including the reduction of wear and friction while drastically increasing areal density of data storage devices. Our study here will lead ...

Multiscale Modeling of Head Disk Interface

Multiscale modeling opens a new paradigm by providing a novel methodology of establishing molecular design criteria and potentially gives several order of magnitude advances in nano-technology. The head disk interface in hard disk drive system investigated here can be used as a benchmark for multiscale modeling. Our approach, stemmed from novel middle-out approach in modern multiscale modeling, uses lattice Boltzmann method (LBM) as a base formulation marches towards continuum level (top) and molecular level (bottom). This approach will be an extremely valuable tool in generating design criteria of head/disk interface.

Multicomponent gas mixture air bearing modeling via lattice Boltzmann method

As the demand for ultrahigh recording density increases, development of an integrated head disk interface (HDI) modeling tool, which considers the air bearing and lubricant film morphology simultaneously is of paramount importance. To overcome the shortcomings of the existing models based on the modified Reynolds equation (MRE), the lattice Boltzmann method (LBM) is a natural choice in modeling high Knudsen number (Kn) flows owing to its advantages over conventional methods. The transient and parallel nature makes this LBM an attractive tool for the next generation air bearing design. Although LBM has been successfully applied to single component systems, a multicomponent system analysis has been thwarted because of the complexity in coupling the terms for each component. Previous studies have shown good results in modeling immiscible component mixtures by use of an interparticle potential. In this paper, we extend our LBM model to predict the flow rate of high Kn pressure-driven flows in multicomponent g...

Multi-scale/multi-physical modeling in head/disk interface of magnetic data storage

The model integration of the head-disk interface (HDI) in the hard disk drive system, which includes the hierarchy of highly interactive layers (magnetic layer, carbon overcoat (COC), lubricant, and air bearing system (ABS)), has recently been focused upon to resolve technical barriers and enhance reliability. Heat-assisted magnetic recording especially demands that the model simultaneously incorporates thermal and mechanical phenomena by considering the enormous combinatorial cases of materials and multi-scale/multi-physical phenomena. In this paper, we explore multi-scale/multi-physical simulation methods for HDI, which will holistically integrate magnetic layers, COC, lubricants, and ABS in non-isothermal conditions.

Temperature Profile in the Presence of Hotspots in Heat Assisted Magnetic Recording

Recently, the demands for increasing memory capacities in hard disk drives (HDDs) has resulted in state-of-the-art technologies including heat assisted magnetic recording (HAMR) with significantly higher operating temperatures. HAMR results in swift degradation of current lubricant and carbon overcoat (COC) materials, leading to magnetic media corrosion which is detrimental to HDD operation. In addition, the lack of thorough understanding of the temperature profiles arising from the hotspot and energy management throughout these materials also exacerbates the problem. To address this issue, in this paper we will focus on the COC and investigate the transient heat transfer in various examples of nanoscale thin films when a hot spot is created via lattice Boltzmann method (LBM) since traditional conduction models like Fourier law are not accurate due to dominant sub-continuum effects. LBM originates from the Boltzmann transport equations (BTEs) and is computationally efficient due to easy parallelization with convenient handling of complex geometries. Our results of the heat transfer mechanism and temperature profiles show that Fourier equation under-predicts the peak temperature rise at the center of the hot-spot as the system size approaches the nanoscale domain. Applying LBM to a multilayered system, we observe a temperature slip along the interface of two materials indicated by the broken isothermal contours, as the heat is confined to a single layer. Using LBM, we then explore a novel graphene overcoat which has outstanding thermo-mechanical properties, and thereby extremely compatible in HAMR applications.

Molecular rheological analysis on binary blends of perfluoropolyether lubricants

The molecular rheology of PFPE becomes critically important in designing optimal lubricants that control the friction/wear and air-bearing by tuning elastic or viscous shear/elongation deformations, which affect the performance and reliability of the hard disk drive. In this paper, we examine the rheological responses of nano blended PFPEs including storage (elastic) and loss (viscous) moduli (G′ and G″), by monitoring the time-dependent-stress-strain relationship via non-equilibrium molecular dynamics simulations. By introducing binary blend of nonfunctional and functional PFPEs, we control the degree of liquid/solid-like behavior using the rheology as a complementary tool for design criteria by tuning molecular conformation and diffusion with nano blend ratio.

Atomistic simulation method in head-disk interface of magnetic data storage systems

The conventional modeling paradigm of head-disk interface (HDI) in magnetic data storage systems was based on meso/macro scale modeling. We investigate inter-molecular interaction energy for four sets of model PFPE dimers and elucidate the importance of hydrogen bonding between the hydroxylated functional endgroups in interaction strength. We found that for these model dimers representing PFPEs, the DDPA-DDPA (non-hydroxylated) dimer demonstrates the least stable interaction. We further investigated binary blends of hydroxylated and non-hydroxylated model PFPE lubricant and observed diminished interaction strength as compared to pure hydroxylated dimers. Our atomistic interaction energy study reported here will provide insight for tuning physiochemical properties of lubricant film by controlling blending ratios and chain end functionality to obtain desired lubricant performance.

Hierarchical Multiscale Modeling Method for Head/Disk Interface

Multiscale modeling opens a new paradigm by providing a novel methodology of establishing molecular design criteria and potentially gives several orders of magnitude advances in nanotechnology. The head/disk interface (HDI) in the hard disk drive system investigated here can be used as a benchmark for multiscale modeling. Our approach, stemmed from the novel middle-out approach in modern multiscale modeling using the lattice Boltzmann method (LBM) as the centerpiece formulation, marches towards continuum level (top) and molecular level (bottom). This approach will become an extremely valuable tool in generating design criteria of HDI.